BACKGROUND: Randomized trials can compare economic as well as clinical outcomes, but economic data are difficult to collect. Linking clinical trial data with Medicare claims could provide novel information on health care utilization and cost. METHODS: We linked data from Medicare claims of women >/=65 years old who had Medicare fee-for-service coverage with their clinical data from the Women's Health Initiative trials of conjugated equine estrogens plus medroxyprogesterone acetate (CEE+MPA) versus placebo and of CEE-alone versus placebo. The primary outcome was total Medicare spending during the intervention phase of the trial, and the secondary outcomes were spending on diseases hypothesized a priori to be sensitive to the effects of hormone therapy. RESULTS: In the CEE+MPA trial, 4,557 participants >/=65 years old were included. Women randomly assigned to CEE+MPA had 4% higher mean Medicare spending overall ($45,690 vs $43,920, P = .08) but 0.5% lower spending for hormone-sensitive diseases ($3,526 vs $3,547, P = .07), with 73% higher spending for coronary heart disease (P = .045) and 122% higher spending for pulmonary embolism (P = .026). In the CEE-alone trial, 3,107 participants were included. Total spending among women randomly assigned to CEE was 3.3% higher ($75,411 vs $72,997, P = .16), and 1.7% higher spending for hormone-sensitive diseases ($5,213 vs $5,127, P = .57), but with 39% lower spending for hip fracture (p<0.03). CONCLUSIONS: Menopausal hormone therapy increased spending for some diseases, but decreased spending for others. These offsetting effects led to modest (3%-4%), nonsignificant increases in overall spending among women aged 65 years and older.